This invention relates to lenses for eyewear, and more particularly to eyewear lenses having small power errors and low geometric distortion.
Spherical-plano lenses are used in a variety of eyewear, such as sunglasses, protective glasses and goggles (e.g., ski goggles, motorcycle goggles). Such eyewear are examples of non-prescription eyewear because they can be obtained without a prescription from an eyecare professional. Spherical plano lenses tend to suffer from small power errors and geometric distortion. As an example, FIGS. 1A and 1D are plots of the mean power error and magnification for an 8 base plano lens. FIG. 1A shows that the mean power of the lens varies from between 0.00 and −0.05 diopters for a zone at the center of lens to between −0.15 and −0.20 diopters for a zone at the lens' periphery. The variation in mean power is shown for a cross-section in FIG. 1B, where “position” refers to radial position in mm, FIG. 1C shows the surface curvature (in diopters) of the front and back surfaces of the lens. FIG. 1D shows the lens' magnification varying from 1.11% (i.e., 1.0111) to 1.12% (i.e., 1.0112) for a zone at the center, up to between 1.13% and 1.14% in at the lens' periphery. FIG. 1E shows a plot of mean magnification as a function of radial position for this lens, and FIG. 1F show a plot of lens thickness as a function of radius. The examples referred to here are 2 mm center thickness, 80 mm diameter polycarbonate lenses. The plotted magnification is the mean equivalent magnification.
It is possible to substantially eliminate one component of either of these errors, for example, mean power error or variations in the mean equivalent magnification, by aspherizing one of the lens surfaces. For example, by aspherizing one of the lens surfaces it is possible to substantially zero the mean power. Referring to FIGS. 2A through 2F, for example, one can optimize the back surface to substantially eliminate the mean power. The result is that the mean power varies by within a range from better than −0.05 diopters to +0.05 diopters across the lens (See, e.g., FIG. 2B, which shows that mean power is close to 0.0 across the lens diameter). However, the variation in magnification across the lens increases significantly (compare FIGS. 1D and 1E with FIGS. 2D and 2E, respectively). The variation in the mean magnification increases significantly, varying across the lens from slightly above 1.00% at the center, to 2.00% or more at the lens' periphery. FIG. 2F shows a plot of lens thickness (in mm) as a function of radius (in mm) for this lens.
Alternatively, it is possible to aspherize a lens surface to make the mean magnification substantially constant over the lens. For example, referring to FIGS. 3A through 3F, aspherizing one surface to minimize the variation in the mean magnification can provide a lens that has a magnification that is between 1.10% and 1.15% across its surface (see FIGS. 3D and 3E). However, evening out the mean magnification increases the variation of the mean power across the lens. FIGS. 3A and 3B show that the lens has a mean power that varies from 0.00 diopter at the center to less than −0.25 diopters at the periphery. Surface curvature as a function of radius (in mm) is shown for this lens in FIG. 3C, and lens thickness (in mm) as a function of radius (in mm) is shown in FIG. 3F. In general, surface curvature (in diopters) is determined as 530 divided by the surface radius of curvature in millimeters.